The present invention concerns a continuously variable transfer drive assembly or transmission mechanism, such as the type suited for use in automotive applications to drive accessory devices. More particularly, the invention relates to a mechanically adjustable belt-type pulley system.
Automotive vehicles include a cooling system to dissipate heat developed by the vehicle power plant, such as an internal combustion engine. In a typical automotive vehicle, the lubrication system provides some cooling function as hot lubricant is pumped away from the engine. However, the bulk of the cooling requirements for the automotive vehicle is accomplished by air flowing through the engine compartment and across a radiator. Coolant flowing around the power plant extracts heat from the engine, which heat is subsequently dissipated through the vehicle radiator.
In automotive vehicles, the engine compartment is designed to permit flow of ambient air through the compartment and past the radiator. In most vehicles, a cooling fan is provided that increases the flow of air across the radiator. In some vehicle installations, the fan is driven by an electric motor that is independent of the vehicle engine. For smaller passenger cars, the electric motor approach can satisfy the cooling needs for the vehicle. However, unlike passenger cars, heavy trucks cannot use electric motors to drive the cooling fan. For a typical heavy truck, the cooling fan would require up to 50 horsepower to cool the engine, which translates to unreasonably high electrical power requirements.
In a typical automotive installation, whether light passenger or heavy truck, the cooling fan is driven by the vehicle engine. In one typical installation shown in FIG. 1, an engine 10 provides power through a drive shaft 11 to a transmission 12. Power to the driven wheels is accomplished through a differential 14. In addition to providing motive power, the engine 10 is also coupled to a transfer drive assembly 15. This assembly 15 provides power directly to a cooling fan 16 that is preferably situated adjacent the vehicle radiator 17.
A wide range of technologies is available to transmit power from the engine 10 to the rotating cooling fan 16. For instance, some transfer drive assemblies 15 are in the nature of on/off clutches. The clutches utilize a friction material to engage the fan when the clutch is actuated. A belt between an output shaft of the engine and the clutch provides rotational input to the clutch in relation to the engine speed. In another drive assembly, a viscous fan drive relies upon the shearing of viscous fluid within a labyrinth between input and output members of the drive. The engagement of the drive is controlled by the amount of fluid allowed into the labyrinth. Viscous drives suffer from many deficiencies. For instance, drives of this type are inherently inefficient because a great amount of energy is lost in heating the viscous fluid. For many viscous drives, this parasitic power loss can be as high as five horsepower.
Another difficultly experienced by viscous fluid fan drives is known as “morning sickness.” When the vehicle is started cold, the fluid in the fan drive is more viscous than under normal operating conditions. This higher viscosity causes the drive to turn the cooling fan at full speed, which causes the cooling system to operate at maximum capacity during a time when the vehicle engine needs to be warming up. A further problem with viscous fan drives is that they require a residual speed even when fully disengaged. This residual speed is usually in excess of 400 r.p.m. and is necessary to allow enough fluid circulation within the drive labyrinth for the drive to re-engage on demand.
The most prevalent transfer drive systems for a vehicle cooling system rely upon a continuous belt to transfer rotational energy from the vehicle engine to the cooling fan. In the simplest case, one pulley is connected to an output shaft of the engine and another pulley is connected directly to the cooling fan. In this simple case, the speed of the cooling fan is directly tied to the engine, varying only as a function of the fixed diameters of the two pulleys. Typically, the ratio of these diameters generates a speed ratio greater than 1:1—i.e., the fan pulley rotates faster than the engine pulley.
One problem exhibited by fixed pulley fan drives is that the fan speed is limited to the fixed ratio relative to the engine input speed. For most vehicles, and particularly most heavy trucks, the maximum cooling air flow requirements occur at the engine peak torque operating condition, which is usually at lower engine speeds. Thus, in order to achieve the proper cooling flow rates, the cooling fan must be sized to provide adequate cooling at the lower engine speeds. The power generated by a fan is related to the cube of its speed. Thus, a fan sized to cool an engine at a lower speed, such as 1200 r.p.m., is grossly oversized at higher engine operating speeds, such as a typical rated speed of 2100 r.p.m. From a cooling standpoint, the significantly greater cooling power provided at higher speeds is not detrimental. However, this over-sizing of the fan equates to wasted power when the engine is not operating at its peak torque condition. For example, a typical 32-inch cooling fan operating at an engine rated speed of 2100 r.p.m., draws approximately 45 horsepower. Of this 45 horsepower, only a fraction, in the range of 10 horsepower, is actually necessary to meet the engines' cooling requirements at this speed.
In order to address the varying cooling needs throughout an entire engine operating range, various cooling systems have been developed. For instance, in one type of system, the blades of the fan are rotated to provide variable flow rates. In another application, the shapes of the fan blades themselves are altered to increase or decrease the flow rate at a constant fan rotational speed.
One approach to solving the problem of varying cooling needs in an automotive setting has been the continuously variable transmission (CVT) or variable transfer drive assembly. In its most fundamental design, the CVT utilizes a continuous belt having a V-shaped cross section. The belt is configured to engage conical friction surfaces of opposing pulley sheaves. The continuously variable feature of the CVT is accomplished by changing the distance between the sheaves of a particular pulley. As the sheaves are moved apart, the V-shaped belt moves radially inward to a lower radius of rotation or pitch. As the sheaves are moved together, the conical surfaces push the V-shaped belt radially outward so that the belt is riding at a larger diameter. The typical CVT is also sometimes referred to as an infinitely variable transmission in that the V-belt can be situated at an infinite range of radii depending upon the distance between the conical pulley sheaves.
Much of the development work with respect CVT's has been in providing a continuously variable transmission between a vehicle engine and its drive wheels. In a few instances, CVT's have been applied as an accessory drive. For example, NTN Corporation has developed a rubber belt CVT system that provides a constant accessory drive speed regardless of engine speed. The system using two spring-loaded adjustable pulleys, each having centrifugal weighs that compensate for changes in engine speed. In this system, as the engine speed increases, the centrifugal weights translate radially outward to exert a force on one sheave pushing it toward an opposing sheave. This change in diameter of the sheave maintains a fixed rotational speed, even as the engine speed increases, by altering the ratio of pulley diameters. This fixed speed is used to maintain a constant alternator speed.
Ideally, a transfer drive assembly, such as assembly 15 shown in FIG. 1, would turn the cooling fan only as fast as is necessary to maintain an optimal engine temperature. Controlling the cooling fan speed conserves power and improves the engine's overall efficiency. In addition, the transfer drive assembly should have the ability to turn the fan faster at lower engine speeds than at higher engine speeds, because the cooling requirements for the engine are greater during operation at low speed and high torque.
Thus far, no accessory drive assemblies are known that are capable of achieving all of these features. Although the continuously variable transmission has been beneficial in operation of cooling fans, the typical CVT cannot accomplish all of these particular factors.